You will be given a definition of a task first, then some input of the task.
In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D". 

[header] How to contrast biochemical reactions inside the body and chemical reactions in the chemistry laboratory [title] Define metabolic reactions. [step] Metabolic reactions refer to biochemical reactions that take place inside the body of living systems. This includes mammalians and microorganisms in addition to plants. <sep> (A) [substeps] Regulate the extent of metabolic reactions to distinguish them from outside or inside the body. Humans are categorized by their biochemical reactions. (B) This type of chemical reactions is characteristic of living tissues only. It is not observed in the chemistry laboratory for example. (C) They are both often called metabolic reactions because they form automatic connections between different bodies of the system. For instance, instead of a human's body producing gas, an electric circuit will create electricity generated from cells. (D) Motor reactions are physical reactions that take place in the body. They contain biological reactions that attack the environment, environmental reactions, and chemical reactions used for physical actions.
Output:
B